Method of treating sheet metal



May 12 936. I Hi. WQNIEMAN 2,040,442

METHOD OF TREATING SHEET METAL Filed Nov. 30, 1.932

reams a 12, 1936 MliTHOD or TREATING snnn'r METAL Henry W. 'Nieman, Bethlehem, la., assignor to Bethlehem Steel Company, a corporation of Pennsylvania Application November 30, 1932, Serial No. 644,972 12 Claims. (01. 153-54) The present invention relates to methods of conditioning mild steel sheets, strips, and analogous thin flat articles for subsequent fabrication into articles of manufacture.

A large quantity of steel sheets or strips having a carbon composition in the neighborhood of {5% are rolled in the manufactureof tinplate, automobile sheetsand the like, the reduction to final gauge being made either by hot or cold rolling. In either case the metal as it leaves the rolls is in a hard and unworkable condition and usually must be given an anneal. This renders the metal soft and ductile but at the same time introduces an undesirable quality, a tendency to flute or 15 form stretcher strains upon subsequent fabrication.

Fluting and stretcher straining of annealed low-carbon sheet material may be designated its peculiar and very undesirable property of yielding locally instead of over a broad area when 20 deformed beyond its yield point. A mild steel sheet which bends sharply at one point instead of curving uniformly over a broader area when subjected to flexing forces is said to flute. A mild steel sheet, the surface of which apparently cracks when the sheet is subjected to a tension beyond its elastic limit, is said to "stretcher strain.

When a sheet of such material is elongated in 30 a testing machine, no permanent set takes place until the very definite yield point is reached. Then a series of apparent cracks develops as the elongation continues, and these cracks, which are known as stretcher strains, broaden out 35 until they finally meet and cover the entire area, which then has a matte appearance quite different from the originally shiny appearance of the sheet. As the elongation is continued from this point on, the'tension increases gradually until the speci- 40 men necks out, and breakage finally results. If the test is halted when the stretcher strains have covered only a portion of the surface, it is found that the material, as measured in a strained area. is thinner than the original sheet. The phenomena of stretcher straining is particularly observed in the fabrication of automobile bodies and similar objects where the stretching of certain parts of the sheet by the dies is sufficient to cause stretcher strains" but not suilicient to cause these to 50 coalesce into a relatively smooth surface. The

resulting pattern is, of course, highly undesirable.

Various methods have heretofore been employed for the removal of the tendency to strain and.

5 flute from a mild steel annealed sheet. Thus. it

has been common to give the sheet, after annealing, a light cold rolling or planishing. While this method of eliminatingthe tendency of the sheet to flute upon fabrication is, in most instances when utilized, capable of producing the desired 5 results, it is not always a practical or desirable method in that the cold rolling operation must be carried out very carefully to avoid warping the sheet or hardening it unduly and also by reason of the fact that a new surface is imparted to the sheet by the rolls, which is often undesirable, for instance where the sheet has been previously tincoated. I v Again, it is known that mild steel annealed sheets may have their qualities improved, insofar as fluting and stretcher straining are concerned, by passing them through roller levelers. It has been found, however, that the use of a roller leveler is not possible in connection with certain strips or sheets because of the very pronounced tendency of the sheets to flute or wrinkle in the leveler, and, moreover, with other steels which do not actually wrinkle therefrequently are formed thereon multitudes of fine parallel lines. These may be too :lne for measurement but still deface the sur- 5 ace.

I have discovered that the tendency of mild steel annealed sheets to flute or to form stretcher strains, during fabrication, may be completely eliminated or removed by subjecting such sheets after annealing to a special hot working. process, the nature of which will hereinafter be set forth in detail.

I have discovered that sheet steel, even though it has a pronounced tendency to flute or form stretcher strains at ordinary room temperatures, can be bent or stretched-smoothly when heated to a point. above. a certain definite critical temperature which is generally between 700 and 750 F. If an annealed sheet of tin plate gauge (and the average tin plate gauge is in the neighborhood of 12/1000 of an inch) is heated to this temperature and bent over a roller of three inches in diameter firstin-one direction and then in the other, while the sheet is cooling, it will be found that when cold the sheet will no longer flute in any direction and will show only extremely shallow stretcher strains when stretched.

As few as four bending operations accomplish this result although it is generally desirable to employ more.- The qualities of the metal will even be considerably improved by two fiexures of the metal at this temperature, the flexures being in opposite directions.

The ability of annealed sheet steel to bend or terial in a further and different manner for a strip or sheet of annealed mild steel is wound,

the elimination of its tendency to flute or form stretcher strains in subsequent'fabrication. Thus, if the metal is stretched while in the heated condition to a limited extent, say 1% or 2% of its total length, it is found that the qualities of the metal are greatly improved; that is, it is found that when the metaLafter it is cooled, is

submitted to fabrication, it will work smoothly and has no tendency to flute or to form stretcher.

strains.

Various types" of apparatus are suitable for carrying on the treatment 'of the metal in accordanee with the-methods above outlined, and my invention is in no wise limited in its broader aspect to any particular apparatus. In the accompanying drawing I illustrate. three forms of apparatus which I have found entirely suitable in practice, but one skilled in the art will readily appreciate that in carrying out the method under varying circumstances the actual apparatus used may be very widely varied in the design and arrangement of its component elements without departure from the invention.

In the drawing: a

Figure 1 shows diagrammatically apparatus for the undulation or flexure of a traveling sheet or strip when in heated condition;

Figure 2 illustrates, also diagrammatically, a suitable apparatus for subjecting a sheet or strip to a stretching operation while in heated condition; and g Figure 3 is a diagrammatic showing of apparatus for the manufacture of tinned sheets, which apparatus includes means for reconditioning the sheets after they have been subjected to the action of the tin pot.

' The apparatus disclosed -in Figure 1 of the drawing comprises essentially a reel l upon which II a furnace, |2-i2 a series of "rollers, and II a second reel or drum upon which the sheet is wound after it passes through furnace Ii and past the rollers I 2. The sheet metal on drum It may be in the annealed state or may be in its hardened condition as it comes from the rolling mill. 'If the metal is already annealed, the heating furnace II will comprise in effect a reheating furnace while, if the has not been annealed, the furnace ll may comprise a continuous heating furnace of the type satisfactory for carrying on the annealing operation. In carrying out the process, the metal is passed through the furnace i I at such speed that it is brought to mately 700, preferably somewhat higher, and then passed directly over the rollers II, at which point it is given four flexures, two in one direction and two in the other. The metal gradually cools during this undulatory treatment and leaves the final roller I! at a comparatively low temperature.

Sheet metal treated by this method is not only smooth working and pliable but suffers practically no impairment of its physical properties by reason of the treatment. It may even show improvement. The exceedingly mild mechanical treat ment it receives does not in any way injure the surface.

In Figure 2 of the drawing, an apparatus for stretching the metal while in the heated condition is shown.

Here the sheet is pulled from the drum 20,

sheet on the drum ill a temperature of approxiin suitable condition for fabrication,

which is provided with a brake to furnish back tension, over the idler cylinder 2! and through a pot of molten salts 22 which furnishes the heat, over the rolls II and 24 and on to the drum II,

which is driven from a suitable sourceof power.

The rolls II and 24 are geared together with a one-to-one ratio, the gears being indicated at 23' and 24', respectively, but the-roll 24 is of slightly greater diameter than the roll 23. As a result of this arrangement, the sheet is stretched as itbridges the gap between'rolls 23 and 24 by the exact amount of the diflerence in surface speeds of these mils. It has been found that an arrangement such as just described by means of which a strip passing through the apparatus is lengthened about 1%",6, will impart a very satisfactory quality to the strip without injuring its surface or physical properties. Sheet steel that has been treated according to the above method or by any other method for the elimination of the tendency to flute .or form stretcher strains will tend to age, that is, will tend to slowly return to its initial unsatisfactory condition. This ageing usually progresses so slowly that it is not an important factor, but, on' the other hand, if the metal is subjected to a higher temperature even. for a very short time, the ageing proceeds with great rapidity, and the metal may be seriously impaired.

After passing through a tin pot, for instance, the temperature of which is in the neighborhood of 600 F., it will be found that the metal, even if previously treated by one of the above methods or by roller leveling or by cold rolling to remove the tendency to flute or form stretcher strains, will partially revert to is original annealed condition. It-has been found that this ageing or return to its original condition can be prevented by passing the strip over a series of rollers immediately after it emerges from the molten tin, with the result that, when cold, it will still work smoothly. As the temperature of the tin is about 600 1 it may seem that this is in contradiction to the statement made at the beginning that a temperature of about 700 F. is necessary for the flrst bending operation. In this case, however, we are dealing with material that has already been treated so that, before it enters the tin pot, it works smoothly.

The results may be stated more generally as follows: low-carbon steel sheets after annealing are subject to stretcher strains and fluting at room temperatures when they are fabricated, but these effects may be eliminated by'either undulating or stretching, commencing at a temperature above 700 methods set forth above. In the event, however, that a sheetor strip has been subjected to such a hot stretching or hot undulating process or any other process which has placed the material that is, any smooth working condition, and is thereafter sub- Jected to the application of heat below 700 F.

in temperature, it is possible to prevent a reversion of the metal to its original state by bending or undulating the same at a temperature below 700 F. This fact makes it possible to first stretch or undulate sheet metal, commencing at a temperature above 700 to eliminate its tendency to flute or form stretcher strains, to thereafter passgthe metal through a tin pot having a temperature in the neighborhood of 600, and to subsequently prevent reversion of the newly heated metal to its original state by undulating the same over a series of rollers immediately after it issues from the tin pot. A suitable apparatus for carrying out this method is diagrammatically shown in Fig. 3 of the drawing. 1

Here a cold rolling mill is indicated at", a continuous annealing furnace at 3|, a tin not at 32', and a series of rollers, for undulating the sheet after emergence from the tin pot, at 33. Rolls 34 may be utilized to give the cold rolled and annealed sheets a "pinch pass to partially, at least. condition them before they enter the tin pot. Instead of rolls 34, however, an undulating machine, such as disclosed in Figure 1, maybe substituted, the object being to completely condition the sheets prior to subjecting them to the tinning operation. As has been before pointed out, passage of the sheet material through the tin pot effects its very rapid ageing which reduces the effect of any previous treatment to remove the tendency to flute or stretcher strain. The undulation of the tinned sheet while it still has a temperature of approximately 600 F. reconditions the same very satisfactorily, howevenand the resulting product is a tinned sheet with good deep drawing properties and without tendency to flute or form stretcher strains.

No satisfactory explanation has been advanced to explain the occurrence of stretcher strains in low carbon, whereas high-carbon steel, alloy steel, and other metals are, so far as I am aware, free from them. Nor is it understood why they do not occur at higher temperatures. A probable explanation is that at the temperatures foundnecessary for hot undulating of annealed sheets,

about 725 n, the slippage due to working the material will follow the grain boundaries almost exclusively so that any working applied to the metal at this temperature will be working the boundary material and not the material of the grains themselves, so that a substantial amount of amorphous material will accumulate between the grains. If mild working is then continued as the metal cools, amorphous material will not only be prevented from reverting to its crystalline state, but more will form, and when cold the grains will all be enclosed in an envelope of such material.

The condition where the grain boundaries of aflne grain steel are strained and rendered more or less amorphous, with the grains themselves unstrained, is an ideal one for subsequent working as it will bend or stretch without fluting or the formation of stretcher strains while at the same time the strength and ductility are unimpaired It will be understood, however, that the theory stated above is diflicult if not impossible to demonstrate conclusively and is ofiered only as a possible explanation of the necessity, which has been clearly demonstrated, of following the process outlined to achieve the desired results.

Where the sheet material is treated by stretch- 'ing, it maybe found desirable to perform the actual elongation in a series of operations rather than in a single one. The first stretching operation is preferably carried out while the section of the sheet under tension has a temperature of about 750 F. There is no upper limit of temperature, but higher temperatures should be avoided as the metal is harder to handle and also oxidizes more freely. The second stretching operation may be carried out with the metal at a' temperature of approximately 600 F., the third operation at 500 F., the fourth operation at 400 F., and the flfth at 300 F. At each operation the metal is elongated approximately 1% of its length. The steps may be performed by one passage of the sheet successively through four stretch! ing machines. Obviously the temperatures advised above need not be exactly obtained in prac-.

into exc l I have also found that tinned sheets-as well as I untinned sheets, may have their propertiesimproved by-hot stretching, and hence I contemplate utilizing, if desired, stretching machines in.-

lieufof undulating machines in treating the sheet metal after itemerges from the tin pot. Treating the metal by working the same. at, elevated temperatures to remove the tendency to flute need not'be' ,done where it is' possible, to fabricate the sheets, etc., directly intofinished articles at such .7

temperatures. Thus, it is quite possible to makecertain articles (such, for instance, as sheet pipes) without fluting the metal, provided'it is maintained at a sufliciently high temperature,

during fabrication, preferably at a minimum temperature of 700 F.

My discovery may be taken advantageof also in another way. Thus, by heating to a temperature of F., or above, a mild steel annealed v sheet or strip coiled upon a drum or the like, the

metal may be unwound from the drum-without without fluting, whereas if the same metal were unwound at ordinary temperature from the coil.

it might be greatly damaged bythe formation'of flutes. To one skilled in'the art it will be apparent also that the process of treating or fabricating the metal while hot may be helpful in connection with many other operations.

Naturally the specific means employed to carry out the process may-be varied considerably in adapting the invention to the treatment of sheets of various thicknesses and compositions, without departure fromthe spirit of the invention.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent is:

1. The method of producing a mild steel sheet with tinned surfaces and good drawing properties, substantially free from tendency to flute or stretcher strain, which comprises cold rolling the sheet to gauge, annealing the same, treating the sheet to lessen its tendency to flute or stretcher strain, passing the same through a tinpot, immediately thereafter, while still at an elevated temperature, submitting the same to deformation and then cooling the same with sufficient promptness to avoid substantial reversion to a fluting or stretcher straining condition.

2. The method of producing a mild steel sheet with tinned surfaces and good drawingproperties, substantially free from tendency to' flute or stretcher strain, which comprises cold rolling the sheet to gauge, annealing the same, treating the sheet to lessen its tendency to flute or stretcher strain, passing the same through a tin pot, immediately thereafter, while still at an elevated temperature, reversely flexing the same and then cooling the same with sufllcient promptness to avoid substantial reversion to a fluting or stretcher straining condition.

3. The method of fabricating mild steel sheet metal which comprises plastically forming the same into manufactured articles while maintaining the temperature of the metal in the range above and adjacent to the minimum temperature at which the metal in its annealed state is free from the tendency to flutev and stretcher strain.

" 4. The method'of steel tinned sheets or the like which includes stretching the sheet immediately after its emergence from the tin pot and before it has cooled and then cooling the same with sufllcient promptness to avoid sub- 'stantial reversion to a fluting or stretcher strain ing condition. g V V 5. The method of the tendency oi sheets andjthe like to flute or stretcher strain in forming operations which comprises submitting the same to deformation while at a-temperature in the range above and adjacent to the minimum temperature at which the -'metal in its while cooling.

is free from the tendency to flute and then cooling the same with suflicient promptness to avoid sub stantial reversion to avfluting or'stretcher straining condition. I

7. The method of uncoilinga coiled strip of mild steel which tends to flute when straightened at ordinary temperatures comprising the step oi uncoiling the strip while within the temperature range of about 700 F. to red heat.

8. The method of eliminating the tendency oi. sheets and the like to flute or stretcher strain which comprises submitting the same to deformation at elevated temperatures and to further deformation during cooling.

' 9. The method 'of eliminating the tendency ofsheets and the like to 'flute or stretcher strainwhich comprises submitting the same to dei.'-

ormation while at a temperature of 700 F. or above and to further deformation at lower tems P ratures.

.10. The method oi eliminating the tendency of sheets and the like to flute orstretcher strain which comprises oppositely flexing the same at elevated temperatures and to further flexing 11. The method of producing a mild steel sheet with tinned surfaces and good drawing properties, substantially ,free from tendencyto flute or stretcher strain, which comprises cold rolling the sheet to gauge, annealing the same, treating the sheet to lessen its tendency'to flute or stretcher strain, passing the same through a tin pot, and immediately thereafter, while still at an elevated temperature, submitting the same to strain and submitting the same to further deformation while cooling.

12. The method of producing a mild steel sheet with tinned surfaces and good drawing properties, substantially free from tendency to flute or stretcher strain, which comprises cold rolling the sheet to gauge, annealing the same, treating th sheet to lessen its tendency to flute or stretcher strain, passing the same through a. tin pot, and immediately thereafter, while still at an elevated temperature, reversely flexing the same and further reversely flexing the same while cooling.

HENRY W. NIEMAN. 

